1. Technical Field
The present disclosure relates to a radio communication apparatus and a radio communication method that detect interference in an interfering wave environment and that improve a decrease in throughput in communication based on an Institute of Electrical and Electronics Engineers (IEEE) 802.11ah standard, in which an orthogonal frequency-division multiplexing (OFDM) scheme is used.
2. Description of the Related Art
So far, a main target of IEEE 802.11 wireless local area network (LAN) standards has been indoor communication. As standards in a physical layer, standards that are mainly aimed at increasing transmission capacity, such as 802.11b (a maximum of 11 Mbps), 802.11a and 11 g (a maximum of 54 Mbps), 802.11n (a maximum of 600 Mbps), and 802.11ac (a maximum of 6.9 Gbps), have been added. On the other hand, as comprehensive examination of smart meters for realizing smart grids begins, there is an increased need for low-rate, long-distance outdoor transmission. Discussions on assignment of frequencies for specified low-power radio communication intended for such use are also continuing. For these reasons, establishment of new communication standards that use a sub-gigahertz band (a frequency band slightly lower than 1 GHz) is now being examined. In IEEE 802.11, too, Task Group ah (TGah; 802.11ah) for examining a wireless LAN standard that uses a frequency band lower than 1 GHz was found in 2010. One of main requirements specifications in TGah (802.11ah) is to realize “a data rate of 100 kbps or higher and a maximum transmission distance of 1 km”.
In IEEE 802.11a and later standards that use an orthogonal frequency-division multiplexing (OFDM) modulation scheme, including TGah (802.11ah), burst transmission is performed after various types of synchronization are established using a preamble at a top of a packet. A preamble includes a short training field (STF; also referred to as a “short preamble”) and a long training field (LTF; also referred to as a “long preamble”). The STF is used for automatic gain control (AGC) or automatic frequency control (AFC) for coarse adjustment. The LTF is used for AFC for fine adjustment or estimation of channel characteristics. After the preamble, signal information for controlling a data field (also referred to as “DATA”) is provided. The signal information is multiplexed by binary phase-shift keying (BPSK) modulation, which is not easily affected by interference.
It is suggested that in TGah (802.11ah), as in the 802.11n standard, in which a 20 MHz channel or a 40 MHz channel can be used, a 1 MHz channel or a 2 MHz channel be used.
For example, it is disclosed in Japanese Patent No. 4413934 that, in IEEE 802.11n, 40 MHz transmission is realized by simultaneously using two adjacent channels, each having a bandwidth of 20 MHz. One of the two channels is called a “primary channel” and used for performing 20 MHz transmission or sharing control information for managing a basic service set (BSS). The other channel is called a “secondary channel” and used as a channel for expanding the band for the 40 MHz transmission.
Here, it is disclosed in Japanese Patent No. 4413934 that if the amount of interference from another system or a wireless LAN included in the secondary channel is large, frames are transmitted using not a 40 MHz bandwidth but the primary channel having a bandwidth of 20 MHz. As a result, more frames are transmitted and received successfully, thereby improving throughput characteristics. The amount of interference is obtained from frame error rates in 40 MHz reception and 20 MHz reception and the reception power of the primary channel and the secondary channel.
In addition, in U.S. Patent Application Publication No. 2013/0107830, a method for determining the primary channel of a new BSS in 802.11ah in the following manner in consideration of overlap between BSSs in an environment in which a 1 MHz channel or a 2 MHz channel can be used is disclosed. That is, the primary channel is determined through channel scanning performed by an access point (AP) or communication with an AP of an existing BSS. In U.S. Patent Application Publication No. 2013/0107830, the primary channel of a new BSS is determined such that the primary channel does not become the same as the secondary channel of an existing BSS. Thus, by notifying all apparatuses of the primary channel of a new BSS before the establishment of the new BSS, a decrease in throughput in an overlap environment can be suppressed.